Onychodus (/ɒˈnɪkədəs/, from Greek meaning "claw-tooth") is a genus of prehistoric lobe-finned fish which lived during the Devonian Period (Eifelian - Famennian stages, around 374 to 397 million years ago). It is one of the best known of the group of onychodontiform fishes. Scattered fossil teeth of Onychodus were first described from Ohio in 1857 by John Strong Newberry. Other species were found in Australia, England, Norway and Germany showing that it had a widespread range.

Onychodus was about 2 to 4 meters in length and was a pelagic animal. Like other onychodontiformes, it had a pair of tooth spirals (parasymphysial tooth whorls) bearing tusk-like teeth.

The most well-preserved specimen of Onychodus has been found in the Gogo Formation of Western Australia giving palaeontologists more information about the structure of the fish. Other species of Onychodus are known only from poor material based on isolated tusks, teeth and scales.

The most characteristic feature is a pair of retractable, laterally compressed tusk whorls at the front of the lower jaw. These were not attached to any other bone, but fit into a pair of deep cavities on the palate and were free to move. The lower jaw was connected with the upper jaw in a way that made the tusk whorl thrust out as a dagger when the head was raised. The upper jaw, containing 30 teeth which decrease in size posteriorly, is well preserved in many individuals. Juveniles have six tusks, while adults have three.

A relatively complete specimen of Onychodus jandemarrai from Western Australia shows that its length was 47 cm long, the head being 10 cm in length with tusks 1.2 cm long. This specimen is only about half the size of larger individuals, since skulls measuring 19 cm in length have been found. However, a single tusk 4 cm long was found, showing that this specimen belonged to an even larger individual. Evidence found of the body reveals that a cross section of this fish would have been oval in shape. On the sides of the body, Onychodus had a series of pores which provided a sensory system that enabled the fish to locate prey and to position itself in narrow spaces. The tail fin is almost symmetrical around the vertebral column. It was rounded slightly and would have been very flexible with a broad sweep producing forward motion. A long fin extends posteriorly, along half the tail fin, forming the second dorsal fin. Evidence of the first dorsal fin is incomplete, but scientists believe that a fossil element found was its fin support. Ventrally, the large anal fin extends back beneath the anterior part of the tail fin. Scales that overlap anteriorly have been found, the smallest being only 5 mm across, and the largest 22 mm.

Onychodus is the type genus of the order Onychodontida and the family Onychodontidae to which it belongs. The family name 'Onychodontidae' was created for Onychodus by the British palaeontologist Arthur Smith Woodward in 1891. The group of onychodontiformes, described in 1973 by the late Dr. Mahala Andrews, was characterized by a highly kinetic skull and tusk-like teeth.

Suggestions by palaeontologist John A. Long refer to a close phylogenetic relationship between Onychodus and the basal lobe-finned fish Psarolepis from China. It is generally acknowledged that Onychodus and Psarolepis are both basal bony fishes, because of the absence of major features that unite coelacanths, lungfishes and tetrapod-like lobe-finned fishes. The position of Onychodus and Psarolepis in the cladogram is outside the major clade of sarcopterygians (lobe-finned fishes), but at a position more derived than actinopterygians (ray-finned fishes).

Onychodus fossils were first described by John Strong Newberry in 1857. The type species is Onychodus sigmoides Newberry, 1857, from either the Columbus Limestone and/or the Delaware Limestone (Middle Devonian) at Milford, Delaware, and Sandusky, Ohio. Some of the specimens were illustrated in Report of the Geological Survey of Ohio, Volume I, Part II in 1873. One of his plates was drawn by geologist Grove Karl Gilbert. Most specimens that Newberry described belong to Onychodus sigmoides, but he also described another species, Onychodus hopkinsi. The name of the species Onychodus sigmoides comes from the word "sigmoid," meaning "S shaped," referring to the shape of the teeth (or tusks) on the parasymphysial whorl. Most species of Onychodus have this basic form of teeth. In 1889, Neweberry described a third species of Onychodus, O. ortoni. Some of the Onychodus specimens examined by J. S. Newberry are in the American Museum of Natural History, and others are in the Orton Geological Museum at The Ohio State University.